During the drilling of an oil well, it is necessary to periodically determine the distance that the well has progressed. Such distance measurement would appear to be a relatively simple task, but in the environment of an oil well, the accomplishment of this measurement is both time consuming and expensive, and the results obtained by conventional measurement techniques are normally far from accurate.
One method presently used to determine downhole distance in an oil well is to measure the length of drill pipe which has been inserted into the well. To accomplish pipe measurement as the pipe is withdrawn from the well to change bits, the length of each stand of pipe is measured by hand using a steel tape. Not only is this method time consuming, but pipe stretch also contributes to the inaccuracy of the measurement so derived. A long length of pipe expands or stretches within a well, but when the pipe is withdrawn and the pressure and tension to which it has been subjected are withdrawn, the pipe will contract. In a deep well, the degree of pipe stretch occuring is significant, and it is impossible to determine the added length contributed to the pipe by pipe stretch downhole when the pipe is withdrawn and measured at the surface.
Similar errors due to stretch occur when attempts are made to make downhole distance measurements by dropping a weighted line or other elongated flexible measuring device into the well. As the line length increases, the resultant stretch error has also been found to increase.
Ideally, the length of a drill pipe should be measured without withdrawing the pipe from the well, for a measurement of this type would measure the actual length of the stretched pipe under tension in the well and would thus provide an accurate measurement of well depth. For example, if a simple roller type distance measuring unit wherein a roller is used to drive a simple mechanical counter could be passed down the interior of a well pipe, an accurate distance measurement might be obtained. Unfortunately, the distance measuring instruments known to the prior art are usually not capable of withstanding the conditions existing within thousands of feet of drill pipe. For example, the drill pipe will be filled with drilling mud under pressure, and such mud constitutes an abrasive and highly destructive agent if it is permitted to contact the mechanical components of a distance measuring unit. If the drilling mud weighs 10 lbs. per gallon, the hydrostatic pressure of the mud will reached 5,200 psi at ten thousand feet. Th viscous mud at these pressures will be forced into the casings of mechanical counters and gears used in prior art measuring units to inhibit operation and destroy the accuracy of any measurement taken.
Even if the destructive action of drilling mud under high pressures is avoided, no mechanical distance measuring unit is available which will provide accurate measurements while traversing the discontinuities in the interior contour of a drill pipe caused by the joints between pipe sections and withstand the impacts which result when the weighted instrument strikes downhole obstructions. Also, known measuring units are not designed to function within a drill pipe while simultaneously facilitating the passage of drilling mud around and past the instrument.
In the past, a drill pipe measuring unit has been designed with some recognition of the environmental hazards which will affect the unit, such unit being illustrated in U.S. Pat. No. 2,245,221 issued to Thomas T. Nudson et al. This patented measuring unit is adapted to descend through the drilling mud within a drill pipe with rotatable counter traction drive gears in contact with the internal surface of the pipe. A counter or registering mechanism is mounted within a sealed chamber, and the pressure within the chamber is equalized with changing external well pressures in the course of movement of the measuring unit to various well depths.
The Nudson et al unit effectively isolates the distance measuring counter from external drilling mud, but has no provision for protecting the drive gears which are essential to counter operation. These drive gears are completely exposed to both the high pressure drilling mud and the cuttings present in the mud, and these cuttings will operate to clog and terminate operation of the drive gears. Additionally, the drive gears of the patented unit drive the counter during both the descent and ascent of the unit within the drill pipe, thereby increasing the likelihood of error in the resulting measurement. If an accurate measurement is achieved during the descent, the results of this measurement can be destroyed if the counter drive gears are jammed by a cutting during the ascent. Finally, if the drive train in the Nudson et al unit is damaged or worn by contact with the drilling mud, it is necessary to either completely replace the entire measuring unit or to laboriously disassemble the unit so that damaged parts can be repaired or replaced.
Consequently, a need still exists for an accurate and durable distance measuring unit which will withstand the environment within a drill pipe and which will effectively measure the length of a drill pipe within a well.